A reciprocating engine requires a crankshaft for converting the reciprocating motion of pistons in cylinders to rotational motion so as to extract power. Crankshafts are generally categorized into two classes: the type manufactured by die forging and the type manufactured by casting. Especially for multiple cylinder engines, the firstly mentioned die forged crankshafts, which are excellent in strength and stiffness, are often employed.
FIG. 1 is a schematic side view of an example of a common crankshaft for a multiple cylinder engine. A crankshaft 1 shown in FIG. 1 is designed to be mounted in a 4-cylinder engine and includes: five journals J1 to J5; four crank pins P1 to P4; a front part Fr, a flange F1, and eight crank arms A1 to A8 (hereinafter also referred to simply as “arms”) that connect the journals J1 to J5 and the crank pins P1 to P4 to each other. The crankshaft 1 is configured such that all of the eight crank arms A1 to A8 are formed integrally with counterweights W1 to W8 (hereinafter also referred to as “weights”), respectively, and is referred to as a 4-cylinder 8-counterweight crankshaft.
Hereinafter, when the journals J1 to J5, the crank pins P1 to P4, the crank arms A1 to A8, and the counterweights W1 to W8 are each collectively referred to, the reference character “J” is used for the journals, “P” for the crank pins, “A” for the crank arms, and “W” for the counterweights. A crank pin P and a pair of crank arms A (including the counterweights W) which connect with the crank pin P are also collectively referred to as a “throw”.
The journals J, the front part Fr, and the flange F1 are arranged coaxially with the center of rotation of the crankshaft 1. The crank pins P are arranged at positions eccentric with respect to the center of rotation of the crankshaft 1 by half the distance of the piston stroke. The journals J are supported by the engine block by means of sliding bearings and serve as the central rotation axis. The big end of a connecting rod (hereinafter referred to as “conrod”) is coupled to the crank pin P by means of a sliding bearing, and a piston is coupled to the small end of the conrod by means of a piston pin. The front part Fr is a front end portion of the crankshaft 1. To the front part Fr, a damper pulley 2 to drive a timing belt, a fan belt or the like is fitted. The flange F1 is a rear end portion of the crankshaft 1. To the flange F1, a flywheel 3 is fitted.
In an engine, fuel explodes within cylinders. The combustion pressure generated by the explosion causes reciprocating motion of the pistons, which is converted into rotational motion of the crankshaft 1. In this regard, the combustion pressure acts on the crank pins P of the crankshaft 1 via the conrod and is transmitted to the journals J via the respective crank arms A connecting to the crank pins P. In this process, the crankshaft 1 rotates while repetitively undergoing elastic deformation.
The bearings that support the journals of the crankshaft are supplied with lubricating oil. In response to the elastic deformation of the crankshaft, the oil film pressure and the oil film thickness in the bearings vary in correlation with the bearing load and the journal center orbit. Furthermore, depending on the surface roughness of the journals and the surface roughness of the bearing metal in the bearings, not only the oil film pressure but also local metal-to-metal contact occurs. Ensuring a sufficient oil film thickness is important in order to prevent seizure of the bearings due to lack of lubrication and to prevent local metal-to-metal contact, thus affecting the fuel economy performance.
In addition, the elastic deformation accompanied with the rotation of the crankshaft and the movements of the center orbit of the journals within the clearances of the bearings cause an offset of the center of rotation, and therefore affect the engine vibration (mount vibration). Furthermore, the vibration propagates through the vehicle body and thus affects the noise in the vehicle and the ride quality.
In order to improve such engine performance properties, there is a need for a crankshaft that is lightweight and is high in stiffness with the ability to resist deformation.
FIG. 2 is a graph indicating a curve showing the pressure in a cylinder of a four-cycle engine. In FIG. 2, when the position of the crankshaft where the crank pin comes to a top dead point in a compression process is considered as a reference (point of crank angel θ of 0 degrees), an explosion occurs immediately after the top dead point in the compression process. Accordingly, the pressure in the cylinder becomes a maximum combustion pressure when the crank angle θ becomes about 8 to 20 degrees. The crankshaft is subjected to the load of pressure in the cylinder (combustion pressure) as shown in FIG. 2, and also subjected to the load of centrifugal force of rotation. The design of the crankshaft aims to improve the flexural rigidity and the torsional rigidity, thereby achieving deformation resistance against these loads, along with weight reduction.
In designing a crankshaft, generally, the main specifications such as the journal diameter, the crank pin diameter, and the piston stroke are firstly determined. The point that can undergo design changes to ensure sufficient flexural rigidity and torsional rigidity after determination of the main specifications is only the shape of the crank arms. Thus, the design of the crank arm shape is an important factor affecting the performance of the crankshaft. Strictly speaking, as described above, the crank arms mean the oval portions connecting the journals and the crank pins to each other and do not include the portions serving as counterweights.
Japanese Patent No. 4998233 (Patent Literature 1) discloses a technique of making recess grooves in the crank pin-side surface and the journal-side surface of each crank arm, in the center, aiming at an increase in flexural rigidity, an increase in torsional rigidity and also a reduction in weight of the crankshaft. The technique disclosed in Patent Literature 1 provides a design method of a crank arm, focusing on a reduction in weight and an increase in stiffness of each crank arm in the state where the crank angle θ is 0 degrees (that is, in the state where the crank pin is in the top dead point in the compression process). In other words, the design method shows how to reduce the weight of the crank arm while achieving a given target value of stiffness in the state where the crank angle θ is 0 degrees. Also, the design method shows how to increase the stiffness of the crank arm while achieving a given target value of weight reduction.
Japanese Patent Application Publication No. 10-169637 (Patent Literature 2) discloses a method for calculating an optimal distribution of mass moments of the counterweights by using the three-moment equation in the Strength of Materials. The technique disclosed in Patent Literature 2 provides a method including approximating a crankshaft to stepped round-bar beams and adjusting the distribution of mass moments of the counterweights in accordance with the stiffness of the crank arms and the mass moments of the crank arms to minimize the loads on the journals. In other words, according to the method, the stiffness of each crank arm is determined by taking a prepared value or in another way, and thereafter, the distribution of mass moments of a plurality of counterweights (for example, eight counterweights in a case of a 4-cylinder and 8-counterweight crankshaft) is adjusted so that the loads on the bearings of the journals can be minimized.